1. Field of the Invention
The present invention generally relates to vapor recovery systems for use in fuel dispenser applications, and, more particularly, to apparatus for detecting hydrocarbon emissions discharged during refueling activity and for regulating the intake flow of pumped vapors using an adjustable valve that is controlled in accordance with the sensed hydrocarbon concentration.
2. Description of the Related Art
The dispensing of fuel into the gasoline tank of a motor vehicle during refueling operations causes the displacement of volatilized fuel vapors by the incoming fuel resulting in their forcible discharge from the tank. These effluent vapor emissions must be captured or otherwise collected to prevent their escape into the surrounding environment as a contaminant. Vacuum-assisted stage II vapor recovery systems serve to recover hydrocarbon vapors displaced from vehicle fuel tanks during fuel dispensing operations. The released vapors are collected using a vapor pump that draws vapor emissions along a vapor recovery line leading to a storage facility where recovered vapors are subject to some form of treatment process such as recycling or combustion.
Optimal efficiency of the vapor recovery system results when vapor is collected at a rate that corresponds as closely as possible to the instantaneous rate of effluent vapor discharge, allowing minimal excess air to be retrieved. Conventional vacuum-assist systems employ apparatus that accomplishes such flow rate control by adjusting the operating speed of the vapor pump to create an equalization between the recovered vapor flow rate and the liquid fuel dispensing rate. These systems rely upon transducers and other sensing devices for measuring the relevant flow rates. However, this approach to flow rate equalization based on flow rate measurements and adjustments to the vapor pump operating speed may not provide the required precision needed to accurately evaluate the compositional content of the discharge environment because no direct measurement is obtained of the concentration of hydrocarbon in the effluent vapor stream. The hydrocarbon concentration is the only true measure of the suitability of an effluent vapor stream for collection and recovery.
The challenge encountered by all such vacuum-assisted vapor recovery systems involves therefore the selection of a suitable vapor monitoring device capable of dynamically sensing the presence of hydrocarbon components and generating a signal that accurately measures the detected hydrocarbon. One limitation experienced by conventional detection apparatus involves an inability to sense hydrocarbon in both its vapor and liquid state. This deficiency is pronounced when the refueling operation occurs during temperature and pressure conditions favorable to the condensation of gaseous hydrocarbon. The failure to adequately remove the hydrocarbon condensate from the detection surface of a sensing device leads to false readings and an overall corruption of the sensing measurement data, resulting in an unreliable control mechanism for regulating the vapor pump.
Implementing changes to the vapor recovery rate by adjusting the vapor pump operating speed itself presents certain disadvantages because it requires continuous variations to the cycling frequency of the vapor pump motor. This aperiodic operation may necessitate at times certain wide-ranging fluctuations in the motor frequency that could lead to excessive wear and eventually premature breakdown.